The present invention relates to continuously variable transmissions in which power is transmitted through pivotally supported traction rollers whose transmission ratio determining pivot position may be varied by hydraulic mechanisms.
In continuously variable transmissions, two toric traction discs are rotatably supported in a housing along an axis opposite one another to define therebetween a toric cavity in which a motion transmitting traction roller is disposed. The traction roller is frictionally engaged with the toric discs in circles of varying diameters depending on the transmission ratio, and is so supported that it can be moved axially to initiate a change in the transmission ratio. Continuously variable transmissions may be used, for example, in some infinitely variable transmissions.
Various mechanical and hydraulic arrangements are known for continuously variable transmissions. In one such arrangement the traction roller is supported by a pivot trunnion which has an opposing support roller received in a partial circular track. Controlled motion of the support roller in the track and movement of the traction roller axially can be induced by slightly tilting the pivot trunnion in a plane normal to the axis of the toric discs during transmission operation.
Mechanical and hydraulic mechanisms which induce this axial motion and produce change in transmission ratios by such tilting are shown, respectively, in U.S. Pat. No. 4,984,312 issued Oct. 23, 1990 to Kraus, and U.S. Pat. No. 4,911,030 issued Mar. 27, 1990 to Kraus. These mechanisms produce a control force which operates upon a projecting control end of the pivot trunnion to provide slight tilting of the trunnion. Thus, as pivot trunnions are tilted by such mechanisms, their support rollers twist and squirm in the partial circular track, and experience significant edge loading and edge wear. Additional friction force results which resists change in the transmission ratio, reduces the transmission efficiency, and prematurely wears the traction surfaces. Load pressure variations and slippage of contact surfaces can result.
Adding to these problems, the contact forces required to engage the various traction surfaces of the traction roller and the toric discs are typically very high. Such contact forces have been supplied by simple mechanical means, such as a preload adjustment screw, shown in the '312 patent, which applies a loading force to the partial circular track generally normal to the axis, while more significant load forces have been applied by axial cam mechanisms known in the art. Larger, variable loading forces have been supplied by hydraulic means, such as hydraulically driven wedges positioned to apply force to the partial circular track, as shown in the '312 and '030 patents. The hydraulic wedges produce higher contact forces needed to handle the larger torques present in high power transmissions. As shown in the '030 patent, the hydraulic load force acts at the same time as the separately applied control force acts to tilt the pivot trunnion and change the transmission ratio. However, while the hydraulic systems produce higher contact forces, the added forces exacerbate many of the problems caused by tilting, such as twisting and squirming of the support rollers, and increased wear, discussed above.
Accordingly, the need remains for improvements in continuously variable transmissions to facilitate transmission ratio change and provide loading forces, while avoiding the above problems.